As selective call network coverage areas grow to meet consumer demand in larger metropolitan areas, the selective call network service provider must necessarily add additional transmitters to provide a larger coverage area. Interference between signals sent from the various transmitters causes difficulty in reception. This interference occurs in areas which can receive transmissions from two or more transmitters.
As pictured in FIG. 1, a selective call terminal 10 encodes a selective call signal and provides the signal to system controller 12 for distribution to multiple transmitting antennas 15, each transmitting antenna having an associated coverage area 20. The system coverage area comprises all of the coverage areas 20. The signal may be provided from the controller 12 to the antennas 15 via radio frequency (RF) link or via hard-wired links, for example the public switched telephone network (PSTN), or a mixture thereof. Therefore, due to differing path lengths and switching equipment, the transmission of the signal from one transmitting antenna 15 may be delayed from the transmission of the signal from another antenna 15 because the signal arrived at the second antenna later than it arrived at the first antenna 15. It is this delay which causes interference in the overlapping coverage areas 25.
To overcome the signal interference due to staggered transmitting times, solutions have been proposed to provide for simultaneous transmission from the transmitting antennas 15. This process is called simulcast transmission.
One simulcast solution, described in European Patent Application 86105017.7, filed Apr. 11, 1986 and published as Publication Number 0 198 448 on Oct. 22, 1986, involves placing large coils called equalization coils in the transmission path from the controller to each transmitter. By manually varying the amount of coil inserted into the transmission path while monitoring the reception in the area of overlapping coverage, the delay between the signal leaving the terminal 10 and arriving a transmitting antenna 15 is adjusted until the signal arrives at all of the transmitting antennas 15 simultaneously. Once the coils have been set to a preferable length utilized, the reception in the overlapping coverage areas should be improved. Yet, the equalization coils do not take into affect the variations in the length of the transmission path when the PSTN is utilized. As is well known in the art, the PSTN service provider can route the call in any manner, at the providers option, as long as it originates and ends at the required locations. Such random rerouting may increase the length of the transmission path from the terminal 10 to the transmitting antenna 15 and/or may insert additional equipment into the transmission path further varying the time the signal arrives at the transmitter.
Another simulcast solution described in European Patent Application 86104869.2, filed Apr. 9, 1986 and published as Publication Number 0 197 556 on Oct. 15, 1986 (Technomen), maintains a relative time between transmissions from base stations by periodic communication between the base stations. The simulcast transmission method of Technomen synchronizes the transmission of the signals from the transmitters by accurate atomic clocks which are periodically adjusted for the relative time. Regrettably, such a system is extremely costly due to the atomic clocks and extremely difficult to maintain simulcast transmission conditions because of the variation of the delay. Periodic correction of the relative time does not improve the transmission of the signal between corrections. Furthermore, the Technomen simulcast transmission system must be periodically shut down for relative time determinations.
Thus, what is needed is a simulcast system capable of synchronizing the transmission of the signal from the transmitters without regard to variations in the length of the terminal-to-transmitter transmission path, yet not requiring highly accurate measure of terminal-to-transmitter delay times requiring expensive atomic clocks.